The z=1.6748 C I absorber toward PKS 1756+237

We report the detection of the 1560 A and 1657 A ground-state C I absorption features in the z(abs)=1.6748 system toward the QSO PKS 1756+237. We find no associated C I* lines with a resulting 3-sigma excitation temperature upper-limit of T(ex) <= 8.54 (+0.65,-0.56) K, which is consistent with the predicted CMBR temperature of T(CMBR)=7.291 K. Because the redshifted CMBR populates the J=1 level and leaves little room for additional local excitation through either collisions or UV pumping, our data place 2-3 times more stringent limits on particle densities and UV fields than existing Copernicus observations of similar column density sightlines in the Milky Way. We also detect several Ni II lines and the weak Fe II 1611 A line. From the Ni/Fe column density ratio we find evidence for dust at a dust-to-metals ratio similar to that seen toward warm Galactic disk clouds. Based on these findings and supported by our Ly-alpha spectrum we propose to reclassify this system as a damped Ly-alpha absorber.

References(0)

Figures(0)

Loading ...